In the surface protection of surfaces resistant to high temperatures, in particular of metallic materials, there are many kinds of coating systems comprising multilayer coatings that substantially increase the life of the underlying material in aggressive environments. Coating systems are known that afford protection against both corrosion and chemical attack. In this respect fluoroplastic coatings are outstanding, since they afford excellent protection against chemicals and are not attacked by either strong acids or solvents. In addition to their well-known mechanical properties such as high resistance to abrasion and good elasticity the thermal properties of the fluoroplastics also allow them to be used just as they are even when prolonged exposure to temperatures up to 260.degree. is involved.
Like other plastics, however, fluoroplastics exhibit permeability to gases, liquids and solutions. In the case of many fluoroplastics this is very pronounced. However, this permeability, which is a big disadvantage for coating material applications, rapidly decreases as the thickness of the layer increases, and is of no practical importance with thicker layers. Hence in coating with fluoroplastics the aim is to have relatively thick layers when attack on the substrate is to be prevented in the case of aggressive gases, liquids or solutions.
In addition vapour diffusion occurs, i.e. gaseous molecules penetrate the plastic layer and attack the underlying material. Here too the problem is solved by a thicker plastic layer.
However, the thickness of the layers that can be used is not unlimited, since the properties of the plastic materials themselves, particularly of the fluoroplastics, must be taken into account, and in addition thicker layers involve substantial increases in costs. Thus a good way of powder coating has been found to be the electrostatic application of fluoroplastics, in which the plastic has an insulating effect, at least from a certain thickness, and then only needs to be melted on. The difficulty with powder coating is that on sintering the fluoroplastic, i.e. in its molten phase and when the layer exceeds a certain thickness, it obeys the laws of gravity and runs off from the underlying material. To counter this it is necessary to rotate the parts to be coated while melting the fluoroplastic layers in the furnace.
Another method used to counter this running off and to enable thicker layers to be applied is to build in mechanical supports, for example carbon fibres, having properties such that they can be applied electrostatically even in thick layers. Despite these supplementary measures known coating systems can only be built up to a maximum thickness of 800 to 1500 .mu.m. This limited thickness of the known coating systems has however generally proved to be inadequate where the environment is particularly aggressive, for example in chemical plant.